1. Field of the Invention
This invention relates to a mechanism for detecting a rotation amount of a diaphragm for use in an optical system such as a camera.
2. Description of the Background Art
A camera such as a still camera, a video camera and the like has a driving actuator which rotatably operates diaphragm blades through a rotating ring. Conventionally, the actuator is also used for detecting a rotation amount of the diaphragm blades of the camera. FIG. 5 depicts a conventional driving actuator which serves for detection of operation of the diaphragm blades. Referring to FIG. 5, a driving actuator 50 includes a rotor 51 and a stator 53. The rotor 51 consists of semi-circular north- and south-polar magnet halves 51a and 51b which are attached together symmetrically around an axis 52 so as to form a rotor of circular shape. The stator 53 is provided, on an inner surface thereof, with a Hall element 54. The Hall element 54 faces a boundary defined between the magnets 51a and 51b and detects change in leakage flux generated from the rotor 51, which is rotatingly driven by energization of a coil 55 disposed on the stator 53.
When the rotor 51 rotates, the leakage flux traversing the Hall element 54 is altered. The amount of rotation of the diaphragm blades is ascertained by detecting change in an amount of the leakage flux which traverses the Hall element.
However, the driving actuator 50 has a small diameter so that only a small peripheral surface area of the rotor 51 moves relative to the element 54. Accordingly, the accuracy of leakage flux detection is lowered. In case that an opening defined by the diaphragm blades has a small diameter, it is difficult to precisely determine the amount diaphragm blade rotation since the detection can be made only within a limited range.